I. FIELD OF THE INVENTION
The present invention relates to rotary diamond drag bits for use in drilling holes in subsurface formations.
More specifically, the present invention relates to rotary diamond drag bits having a multiplicity of compound polycrystalline diamond compact (PDC) cutting elements strategically mounted on the cutting face of the drag bit.
The compound PDC cutter has a dome shaped PDC trailing surface that acts as a penetration limiter for the leading rock shearing PDC cutter disc associated therewith. The domed trailing surface limits impact damage to the shear cutter disc when drilling hard laminar or non-homogeneous rock formations.
II. DESCRIPTION OF THE PRIOR ART
There are a number of diamond drag bit patents that bear a slight semblance to the present invention, but all are intended to be backup systems for the primary PDC cutters when the cutters become badly worn or broken.
U.S. Pat. No. 4,823,892 teaches embedding small natural diamond or other ultra hard particles in the PDC cutters tungsten carbide surface trailing the primary PDC cutter disc. This concept is designed to be a backup system to abrade away the rock formation after the primary PDC cutter disc is badly worn or broken. Although this does allow some bits to continue drilling, the rate of penetration is extremely slow because the imbedded diamond particles are not aggressive enough to remove nearly as much rock formation per bit revolution as a sharp PDC cutter disc. The embedded diamonds trailing the PDC cutter are very brittle and break under relatively low impact loads, therefore they don't serve as good shock absorbers to prevent PDC cutter breakage.
The present invention overcomes the shortcomings and disadvantages of the foregoing prior art by providing a very strong partial dome shaped polycrystalline diamond layer chemically and metallurgically bonded to the tungsten carbide stud immediately behind the leading PDC cutter disc. This polycrystalline diamond partial dome functions as a depth of penetration limiter and hence a shock absorber for the PDC cutter to minimize cutter damage while drilling. The smooth diamond surface having an extremely low coefficient of friction generates a very small amount of heat while rubbing on the rock formation while drilling. Therefore, heat deterioration of the diamond cutter or dome is of no consequence.